A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet radiation (EUV) having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that radiation with a wavelength of less than 10 nm could be used, for example 6.7 nm or 6.8 nm. In the context of lithography, wavelengths of less than 10 nm are sometimes referred to as ‘beyond EUV’, or as ‘soft x-rays’.
Extreme ultraviolet radiation and beyond EUV radiation may be produced using a discharge produced plasma radiation source. A plasma is created by, for example, passing an electrical discharge through a suitable material (e.g. a gas or vapour). The resulting plasma may be compressed (i.e. be subjected to a pinch effect), at which point electrical energy is converted into electromagnetic radiation in the form of extreme ultraviolet radiation (or beyond EUV radiation). The radiation emitted from the discharge produced plasma radiation source may be collected using a collector, such as a mirrored grazing incidence collector, which receives the extreme ultraviolet radiation and focuses the radiation into a beam.
It has been observed that discharge produced plasma radiation sources produced ions with kinetic energies up to 75 keV (such ions sometimes being referred to as ‘high energy ions’ or ‘fast ions’). For example, a discharge produced plasma radiation source that has tin electrodes has been observed producing fast tin (Sn) ions with energies up to 75 keV. Ions having energies up to 75 keV are known to sputter objects that they come into contact with, for example the grazing incidence collector referred to above as receiving and focusing into a beam radiation emitted from the discharge produced plasma radiation source. Sputtering of the grazing incidence collector may affect the lifetime of the collector. It is desirable to reduce the sputtering of the collector, and therefore increase its lifetime. This is because it is desirable to minimize the costs associated with replacing or repairing a collector, and also the down-time in the use of a lithographic apparatus associated with the repair or replacement of the collector.
Existing discharge produced plasma radiation sources, or structures to be used in conjunction with those sources, often employ a debris mitigation scheme. The debris mitigation scheme is employed to reduce the number of particles, ions, etc. that pass from the radiation source into and around the lithographic apparatus. For example, known debris mitigation schemes are employed to reduce the number of fast ions that are incident upon and sputter the grazing incidence collector. Existing debris mitigation schemes utilize a static foil trap filled with argon. However, in order to effectively suppress the passage of fast ions having energies of around 75 keV, the pressure of the argon gas should be at a level which is technically difficult to achieve and sustain. Furthermore, the desired increase in the density of the argon gas decreases the overall transmission of EUV radiation from the discharge produced plasma radiation source through to the lithographic apparatus.
It is therefore an object of the present invention to provide a discharge produced plasma radiation source, and a method of generating radiation using a discharge produced plasma radiation source which reduces the number of fast (high energy) ions that are generated. Desirably, the reduction in the number of fast ions that are generated is greater than any corresponding reduction in the generation or transmission of radiation (e.g. EUV radiation) generated by the discharge produced plasma radiation source.